i think the plan from the beginning was that eevee would have an evolution for every element it says in the bio that because its genetic makeup is irregular, it quickly changes its form due to a variety of causes.

(WARNING: POSSIBLE SPOILERS AHEAD if you watch Pokemon Black & White Best Wishes Season 2)Sometime during the Isshu (Unova) League, the Eevee Rescue Team guy (can't remember his name) is battling someone with his only un-evolved Eevee, that Eevee takes a hit or something (this new Eeveeloution may need a stone), and when she (that Eevee's a girl) is trying to get up to finish the battle, it evolves into Ninfia and kicks the other Pokemon's ass.

Note: This has not happened. This is just speculation, and I think it's a pretty good theory.